Reaction channels, radical

Indeed, by using soft El ionization, we have been able to unambiguously detect products from all five reaction pathways (2a)-(2e), determine their branching ratio and characterize their dynamics.34 Here we discuss some of the results that we have obtained on this reaction, which well exemplify the power of soft El ionization. First of all, from measurements of the El efficiency curves at various to/e ratios (15, 42, and 43), we have found that the parent ion at m/e = 43 (CH2CHO+, corresponding to one of the main reaction channels, the vinoxy radical,) is not stable, so measurements of angular and TOF distributions were carried out at m/e = 42. Incidentally, from the El ionization efficiency curve at m/e = 42 we have obtained some direct information on the IE of the vinoxy radical, for which no such information was available till now. The IE should be <11 eV. 

Fig. 19. The unimolecular reaction channels for the propargyl radical together with the heats of reaction. Assuming that H2 loss is associated with a substantial reverse barrier, formation of cyclopropenylidene, c-C3H2, is the most likely channel. (From Deyerl et a/.143)...

Jahn combined the formation of the enolate 2-713 resulting from an intermolecu-lar Michael addition of 2-711 and 2-712 with a radical reaction (Scheme 2.157) [363]. The enolate 2-713 did not undergo any further transformations due to the lack of appropriate functionalities. However, after formation of a radical using a mixture of ferrocenium hexafluorophosphate (2-714) and TEMPO, a new reaction channel was opened which afforded the highly substituted cyclopentene 2-715a diastereoselec-tively. 

The pronounced tendency of radicals to engage in one-electron transfer reactions is well documented [3]. This reaction channel is favored because it provides the simplest way for radicals to lose their radical nature, i.e. to b ome species with an even number of electrons (closed-shell molecules). The direction of the electron flow between the radical X and the molecule Y depends on the oxidizing or reducing power of X and on the ability of Y to either donate or accept an electron the final result of the interaction between X and Y is then the either one-electron-reduced or -oxidized former radical (X" or X ) or the open-shell molecle (Y or Y"" ), (cf. Eq. 1) ... 

The concept of this method is illustrated in Scheme 3.1, where the clock reaction (U R ) is the unimolecular radical rearrangement with a known rate constant ( r)- The rate constant for the H atom abstraction from RsSiH by a primary alkyl radical U can be obtained, provided that conditions are found in which the unrearranged radical U is partitioned between the two reaction channels, i.e., the reaction with RsSiH and the rearrangement to R. At the end of the reaction, the yields of unrearranged (UH) and rearranged (RH) products can be determined by GC or NMR analysis. Under pseudo-first-order conditions of silane concentration, the following relation holds UH/RH = (A H/A r)[R3SiH]. A number of reviews describe the radical clock approach in detail [3,4]. 

Once the actinic fluxes, quantum yields, and absorption cross sections have been summarized as in Table 3.19, the individual products < .,v(A)wavelength interval can be calculated and summed to give kp. Note that the individual reaction channels (9a) and (9b) are calculated separately and then added to get the total photolysis rate constant for the photolysis of acetaldehyde. However, the rate constants for the individual channels are also useful in that (9a) produces free radicals that will participate directly in the NO to N02 conversion and hence in the formation of 03, etc., while (9b) produces relatively unreactive stable products. 

The mechanism of decomposition of the Criegee intermediates is believed to occur via several reaction channels shown for the [(R,CH2)(R2)CHOO] Criegee intermediate in Fig. 6.4. The oxygen-atom elimination channel for simple alkenes is not believed to be important. However, the ester and hydroperoxide channels are important and explain the production of free radicals such as OH. Theoretical calculations have shed some light on this (e.g., Gutbrod et al., 1996, 1997a ... 

The rate of reaction (2) is significant in ambient air, even though its rate constant is only 1.5 x 10-15cm3 molecule-1 s-1 at 298K [5,6]. Also, the self-reaction of the radical product H02 [6], i.e., 2H02 - H202 + 02, could be kept negligible in these experiments, so that the relative importance of the three primary reaction channels could be derived from the product distribution... 

HO-initiated oxidation of the alkanes become complex with increase in carbon number. Namely, a large variety of alkyl radicals can be produced by the H-atom abstraction from the primary, secondary and tertiary C—H bonds in the parent alkane [88]. The resulting ROO ( C4) radicals have been shown by Atkinson et al. to yield R0N02 as well as RO + N02 upon reaction with NO [100-102]. A major complication in the alkane oxidation mechanism arises from the variety of competitive reaction channels that RO radicals can undergo, e.g., 02-reaction, unimolecular dissociation and internal isomerization. There have been a number of experimental and theoretical studies of these reactions [31,88]. 

More reaction pathways are opened up when one of the substituents at silicon is a vinyl group as in 116 (equation 28)70. The intermediate silaallylic radical 117 (alternatively, the authors suggest an ionic mechanism) can be trapped directly by methanol to give the silyl ether 118. Alternatively, 117 closes to the silene 119, which was identified by its reaction product with methanol, 120. A third reaction channel is the elimination of a silene, which again was identified by its trapping product 121 with methanol. 

The uncatalysed Belousov-Zhabotinsky (B-Z) reaction between malonic acid and acid bromate proceeds by two parallel mechanisms. In one reaction channel the first molecular products are glyoxalic acid and carbon dioxide, whereas in the other channel mesoxalic acid is the first molecular intermediate. The initial reaction for both pathways, for which mechanisms have been suggested, showed first-order dependence on malonic acid and bromate ion.166 The dependence of the maximal rate of the oxidation of hemin with acid bromate has the form v = [hemin]0-8 [Br03 ] [H+]12. Bromate radical, Br02, rather than elemental bromine, is said to play the crucial role. A mechanism has been suggested taking into account the bromate chemistry in B-Z reactions and appropriate steps for hemin. Based on the proposed mechanism, model calculations have been carried out. The results of computation agree with the main experimental features of the reaction.167... 

In this case, the predicted rate law includes two reaction channels - both first order in phenol, with one first order in radical concentration and the other zero order. This behaviour has been found, for example, in the reaction between 2,2 -methylene-bis(4-methyl-6-terf-butylphenol) (ArOH) and 2,2-diphenyl-1-picrylhydrazyl (Y ) in Fig. 4.7 when acetonitrile, benzonitrile, acetone, cyclohexanone or DMSO are the solvents [19]. 

On the contrary, most homolytical free-radical processes display formation of reactive intermediates capable of reacting by various reaction channels, which reduces the industrial value of these processes. 

Both steps are spin-allowed and exothermic reactions. A scan of the N4 potential energy surface at the CAS(12,12)/cc-pVTZ level indicated that a perpendicular approach of N(2D) towards the molecular plane of 15 is most favorable for formation of 1. This can also be rationalized from an analysis of the occupied orbitals in the two species. According to the computed dissociation pathway at the MRCI-SD(Q)/cc-pVTZ//CAS(15,12/cc-pVTZ level, 1 can be formed from 15 and N(2Z>) (Eq. 2) in a near barrierless process, as is expected for a radical recombination reaction. However, the scan of the potential energy surface shows that the reaction channel is rather narrow and that two N2 molecules is likely to be formed in a competing process. 

The reaction of peroxy radicals with HO2 radicals can proceed via two reaction channels to give either organic hydroperoxides or carbonyl products. 

---